This invention relates to a field effect transistor (FET) and more particularly to a field effect transistor that is capable of improving .alpha. particle immunity or soft error immunity.
A typical structure of a GaAs MESFET device or element, which is a kind of FET using compound semiconductor, is illustrated in cross section in FIG. 1. As seen from the figure, in a semi-insulating GaAs substrate are provided an n type active layer 2, an n.sup.+ type source region 3 and an n.sup.+ type drain region 4 on which a gate electrode 7, a source electrode 5 and a drain electrode 6 are provided, respectively. The semi-insulating substrate is used for the purpose of reducing the parasitic capacitance generated between the FET device and the substrate, thereby realizing a high speed FET device. The FET device made directly on such a semi-insulating substrate is disclosed in e.g. an article by R. C. Eden in the Proceedings of the IEEE. Vol. 70, No. 1, pp 5-12 (Jan. 1982).
In the memory device, such as an SRAM (static random access memory) formed with such FETs, the stored information can be destroyed when .alpha.-particles are incident into the substrate. This phenomenon, called "soft error", was found by T. C. May and M. H. Woods in 1979 in the memory device fabricated using Si semiconductor elements (see IEEE Transaction Electron Device, ED-26, p2, 1979).
The mechanism of producing the soft errors in the memory device constituted by Si semiconductor elements are generally considered as follows. The memory information is held by the amounts of charge stored in the capacitors located at respective memory cells of a memory device in case of DRAM (dynamic random access memory), or held by the electric potentials at the nodes in respective memory cells in case of SRAM. When an .alpha. particle is incident on the memory cells, about 10.sup.6 pairs of electrons and holes are generated along the track of the .alpha.-particle in the Si substrate. These carriers diffuse or drift in the substrate to flow into the capacitors or nodes. Thus, the amounts of charge stored in the capacitors and the electric potentials at the nodes are greatly varied to destroy the stored information.
If 10.sup.6 pairs of the carriers are generated in total along the track of an .alpha. particle in the memory device constituted by the Si semiconductor elements, the total amount of charge flowing into the capacitors or nodes is 160 fC (1.6.times.10.sup.-19 .times.10.sup.6 =160.times.10.sup.-15 C) at most assuming that all the carriers generated flow into there, and never exceeds 160 fC.
It had been considered that the soft errors are generated through a similar mechanism also in the SRAM constituted by the GaAs MESFET's made directly on the semi-insulating substrate. The inventors, however, found the following fact as a consequence of repeated measurements of the charge amount in the transistor operation of the GaAs MESFET's made directly on the semi-insulating substrate. The fact is that unlike the Si memory device, the incidence of an .alpha. particle appears to produce the charge amount several times as large as 160 fC which flows into the electrodes of MESFET's. This fact means that the memory device constituted by the GaAs MESFET's made directly on the semi-insulating substrate exhibits .alpha.-particle immunity lower than the Si memory device. This is considered to be caused by the reason that the memory device formed with GaAs MESFET's made directly on the semi-insulating substrate offers a mechanism of multiplying the carriers, i.e. electrons or holes, in response to the incidence of an .alpha. particle.
The inventors of this invention found the following carrier multiplication mechanism. The total charges of holes and electrons produced in the GaAs substrate upon the incidence of an .alpha. particle are respectively 160 fC and are equal. In the state where the drain is supplied with a voltage, electric lines of force run from the drain to the source and they facilitate the generated carriers to drift. Regarding the mobility that determines the drift speed of the carriers, the electron mobility is more than ten times as large as the hole mobility in GaAs. Therefore, even after the electrons re completely absorbed from the substrate to the drain, the holes remain in the substrate. Thus, the potential barrier for the electrons at the substrate side becomes low and the injection of electrons from the source side is facilitated. Thus, subsidiary current path is formed and so a current newly flows into the drain. The total amount of charge thus flowing into the drain becomes several times as large as 160 fC, which is observed as a carrier multiplication phenomenon. Therefore, it is considered that such a carrier multiplication mechanism can be suppressed by inducing the status where the holes don't remain in the substrate, i.e. a state where electrons are prevented to rapidly flow into the drain.
In this way, in order to prevent the soft error due to the incidence of .alpha.-particles in the memory device constituted by GaAs MESFET's made directly on the semi-insulating substrate, the above carrier multiplication mechanism as well as the mechanism common to the Si substrate must be taken into consideration.
In order to prevent the soft error due to the incidence of .alpha. particles, it has been proposed to form, below an active layer of the FET, an impurity layer having a conductivity type opposite to that of the active layer. This method, however, only prevents the intrusion of either the electrons or the holes generated because of the incidence of .alpha.-particles, and also is insufficient for preventing the above carrier multiplication mechanism.
A related prior art is also disclosed in JP-A-57-211783.